Our research program centers on achieving a comprehensive molecular understanding of human polycystic liver and kidney disease. Our goal is discovery novel paradigms for effective treatment of patients. We have taken an approach that begins with disease gene discovery through positional cloning to understand the genetic bases for the diseases, followed by functional studies to understand cellular pathogenesis. Under the current grant, we recruited patients with isolated autosomal dominant polycystic liver disease (ADPLD; MIM 174050) and completed a comprehensive clinical characterization of the disease. We established genetic linkage for one locus, PLD1, and discovered the disease gene, PRKCSH. We identified a second locus for ADPLD on chromosome 6 and identified human SEC63 as PLD2. Mutations in these genes account for ~ 25% of ADPLD probands in our cohort and suggest that there is at least one additional locus, PLD3, for ADPLD. PRKCSH encodes the B-subunit of glucosidase II (GIIB), a glucose trimming enzyme involved in protein maturation and quality control in the ER. SEC63 encodes a component of the ER protein translocation machinery that functions in concert with GII to achieve proper topology and folding of integral membrane or secreted glycoproteins. This proposal seeks to further define cellular pathways to cyst formation by discovery of additional genes involved in ADPLD, by determining whether two hits are required for cyst formation in ADPLD, and by defining the relationship between protein maturation in the ER and the normal function of primary cilia in bile duct epithelium. We propose to identify additional genes (e.g., PLD3) responsible for ADPLD by use of a combination of "classical" positional cloning and candidate gene approaches. We will use mouse models to test the hypothesis that cyst formation in ADPLD requires somatic second hits as is the case in ADPKD. We will determine whether Prkcsh and Sec63 mutations result in improper maturation and cilial delivery of the Pkd 1, Pkd2 and Pkhd 1 gene products. These studies will improve our understanding of the cellular and molecular bases of polycystic diseases by using the novel entry points into the pathogenic pathways made possible by our discovery of PRKCSH and SEC63 as disease genes. At the same time, we will improve our understanding of ADPLD as a disease condition and develop insights that will help patients affected by this condition.